1. Field of Endeavor
The present disclosure relates to a PLC (Programmable Logic Controller), and more particularly to a PLC (Programmable Logic Controller) extension system.
2. Background
Although a factory system used to be operated by independent manipulation of machines and/or devices in automation facilities at a traditional industrial site, changes in the system are being required these days as modern industries become more complicated and diversified. That is, a device is required to complement the complicated and diversified operation, and therefore, a PLC (Programmable Logic Controller) system for directly controlling sites has been developed. To solve the difficulties, a programmable logic controller (PLC) is typically employed. The PLC generally includes a basic module and a wide variety of expansion modules.
That is, industrial automation and control systems are known and are in use for controlling factory automation and the like. Such systems typically include various components that cooperate to monitor and control a process and/or system (e.g., a manufacturing process or a machine). A programmable controller is typically a central feature of a control system. For example, a typical control system includes one or more PLCs (programmable logic controllers) that coordinate with sensors, semiconductor power electronic circuits, power supplies, motor starters, relays, and so forth to control features and actions throughout an associated system or process. In operation, a typical PLC (programmable logic controller) examines a series of inputs reflecting the status of a controlled process and changes outputs affecting control of the process. For example, a programmable logic controller may receive an analog signal from a sensor and adjust an output to an actuator depending on a value of the signal and associated logic programmed in a memory of the programmable controller.
A programmable logic controller (PLC) applied to various fields such as automation of factory facilities in industries may include a base, power module, a central processing unit (CPU) module including a CPU, an input module that receives a signal inputted from a sensor or switch, a high-speed counter module that receives a signal inputted from an encoder, an output module that transfers a control signal to a motor or valve as a control object, a communication module, a proportional integral differential (PID) control module, a positioning module, and the other modules.
One base may be typically mounted with one power module, a communication module in a scope allowable by a CPU module and the base, an input/output module and a special module. Furthermore, one base may be mounted with one power module, which means that the number of modules mountable at the base may be restricted by the power suppliable by the power module. A PLC extension system using a basic base (10) and extension bases (11 to 1N) is provided to solve the restriction and to effectively extend the system as illustrated in FIG. 1.
The FIG. 1 illustrates a PLC extension system according to prior art. All extension bases are connected by daisy chain configuration using extension cables (21, 22, 23, 2N) started from the basic base (10). A PLC network extension system, as illustrated in FIG. 1, is formed by the basic base (10) and n numbers of extension bases (11 to 1N) that are mutually connected.
The PLC network extension system of serial cable type according to the prior art is advantageous in that buses are directly connected by cables to directly control modules of extension bases (11 to 1N) from a CPU module of the basic base (10). However, there is also a disadvantage in that only the extension bases can be controlled at a proximate distance because of restriction in connecting the cables.
That is, there is a severe restriction in distance because a parallel signal transmission is employed using extension cables. The extension cables according to the prior art are formed in a bundle of lines for transmitting a parallel signal necessary for extension base, and the parallel signal that is transmitted via cables may create a restriction in distance due to electromagnetic problem.
Concomitant with sophistication of modern industries and large volume properties of information, flexible extension of PLC system and calculation speed have come to be very important. However, the PLC network extension system according to the prior art thus described has a disadvantage in that there is a distance restriction and complexity of installation, set-up and programming as a programmer is required to understand the network system. Another disadvantage is that network load increases due to control message of main CPU to greatly reduce performance of the scan-based control system as system grows extended.
Accordingly, there is room for improvement in the PLC network extension system.